1. Field of the Invention
This invention relates to electrophotographic photosensitive members and more particularly to an electrophotographic photosensitive member having a photosensitive layer containing a specific hydrazone compound which is a novel organic photoconductive substance.
2. Description of the Prior Art
Inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide, etc. have so far been known as photoconductive materials used for electrophotographic photosensitive members. Contrary to their many advantages, as chargeability to a suitable potential in the dark, high retention of charge in the dark, and fast dissipation of charge on exposure to light, these photoconductive materials have various disadvantages, for instance, as follows: selenium type photoconductive materials readily crystallize under the influence of such factors as heat, moisture, dust, pressure, and the like; in particular, when the surrounding temperature exceeds 40.degree. C., their crystallization becomes remarkable, resulting in deterioration of their chargeability or white spots in images formed therewith. Selenium type or cadmium type photoconductive materials lack stable sensitivity and durability for use under high humidity conditions. Zinc oxide type photoconductive materials, which require sensitization with a sensitizing pigment including Rose Bengal as a typical example, cannot give stable images over a long period of time because the sensitizing pigment tends to be degraded by corona charging and faded by light exposure.
On the other hand, there have been offered various kinds of organic photoconductive polymers including polyvinylcarbazole and the like. These polymers, however, have been hardly put to practical use until now, in spite of their better film-forming property and their lightweight, as compared with the foregoing inorganic photoconductive materials. The reason for this is that their film-forming property is still unsatisfactory and inferior to the inorganic photoconductive materials in sensitivity, durability, and stability to changes in environmental conditions.
Further, low-molecular organic photoconductive materials have been offered including hydrazone compounds and the like disclosed in U.S. Pat. Nos. 4,150,987 and 4,278,747, German Patent Offen No. 2939483, British Patent Laid-Open No. 2034493, and European Patent Laid-Open No. 13172. Although the insufficient film-forming property which has been a problem in the field of organic photoconductive polymers can be overcome by using these low-molecular organic photoconductive materials with a properly selected binder, their initial potential on charging is low and dark decay thereof is significant. In addition, they are inferior in the ability to restore original charge bearing characteristics when they are put again in the dark after being retained in the light for a given time (this restoring power is referred to as "photomemory property"). These drawbacks cause various problems.